Martin Parent

Organization of the basal ganglia: the importance of axonal collateralization

Recent neuroanatomical data obtained with single-axon or single-cell labeling procedures in both rodents and primates have revealed the presence of various types of projection neurons with profusely collateralized axons within each of the major components of the basal ganglia. Such findings call for a reappraisal of current concepts of the anatomical and functional organization of the basal ganglia,which play such a crucial role in the control of motor behavior. The basal ganglia now stand as a widely distributed neuronal network, whose elements are endowed with a highly patterned set of axon collaterals. The elucidation of this finely tuned network is needed to understand the complex spatiotemporal sequence of neural events that ensures the flow of cortical information through the basal ganglia.

Maladaptive plasticity of serotonin axon terminals in levodopa-induced dyskinesia.

Striatal serotonin projections have been implicated in levodopa‐induced dyskinesia by providing an unregulated source of dopamine release. We set out to determine whether these projections are affected by levodopa treatment in a way that would favor the occurrence of dyskinesia.

Two types of projection neurons in the internal pallidum of primates: single-axon tracing and three-dimensional reconstruction.

The axonal projections of the internal pallidum (GPi) in cynomolgus monkeys (Macaca fascicularis) were studied by labeling small pools of neurons with biotinylated dextran amine. Fifty‐two axons were reconstructed entirely from serial sections with a camera lucida. Two types of projection neurons were identified in the GPi on the basis of their target sites. The abundant and centrally located type I neurons gave rise to a long axonal branch that descended directly to the pedunculopontine tegmental nucleus, where it arborized discretely. Other branches ascended to the thalamus and broke into 10–15 thinner collaterals that ran through most of the ventral anterior nucleus, where they terminated as typical plexuses. About half of these axons gave rise to collaterals that arborized in both components of the centre médian/parafascicular thalamic complex. The less numerous and peripherally located type II neurons had an axon that climbed the rostral thalamic pole, coursed along the stria medullaris, and arborized profusely within the lateral habenular nucleus, which stood out as the most densely innervated pallidal target. Some type II axons provided collaterals to the anterior thalamic nuclei. A small proportion of axons of both types had branches that crossed the midline and terminated in contralateral GPi target structures. Three‐dimensional reconstruction showed that type I axons arborized principally along the sagittal plane. These data reveal that GPi neurons of type I act through a widely distributed axonal network upon thalamic and brainstem premotor neurons, whereas type II neurons act in a much more focused manner upon lateral habenular neurons. J. Comp. Neurol. 439:162–175, 2001.

Neural transplants in patients with Huntington's disease undergo disease-like neuronal degeneration

The clinical evaluation of neural transplantation as a potential treatment for Huntingtons disease (HD) was initiated in an attempt to replace lost neurons and improve patient outcomes. Two of 3 patients with HD reported here, who underwent neural transplantation containing striatal anlagen in the striatum a decade earlier, have demonstrated marginal and transient clinical benefits. Their brains were evaluated immunohistochemically and with electron microscopy for markers of projection neurons and interneurons, inflammatory cells, abnormal huntingtin protein, and host-derived connectivity. Surviving grafts were identified bilaterally in 2 of the subjects and displayed classic striatal projection neurons and interneurons. Genetic markers of HD were not expressed within the graft. Here we report in patients with HD that (i) graft survival is attenuated long-term; (ii) grafts undergo disease-like neuronal degeneration with a preferential loss of projection neurons in comparison to interneurons; (iii) immunologically unrelated cells degenerate more rapidly than the patients neurons, particularly the projection neuron subtype; (iv) graft survival is attenuated in the caudate in comparison to the putamen in HD; (v) glutamatergic cortical neurons project to transplanted striatal neurons; and (vi) microglial inflammatory changes in the grafts specifically target the neuronal components of the grafts. These results, when combined, raise uncertainty about this potential therapeutic approach for the treatment of HD. However, these observations provide new opportunities to investigate the underlying mechanisms involved in HD, as well as to explore additional therapeutic paradigms.

Axonal branching pattern of neurons of the subthalamic nucleus in primates

Axonal projections arising from the subthalamic nucleus (STN) in cynomolgus monkeys (Macaca fascicularis) were traced after labeling small pools (5–15 cells) of neurons with biotinylated dextran amine. Seventy‐five single axons were reconstructed from serial sagittal sections with a camera lucida. Most of the STN labeled cells displayed five to eight long, sparsely spined dendrites that arborized mostly along the main axis of the nucleus. Based on their axonal targets, five distinct types of STN projection neurons have been identified: 1) neurons projecting to the substantia nigra pars reticulata (SNr), the internal (GPi) and external (GPe) segments of the globus pallidus (21.3%); 2) neurons targeting SNr and GPe (2.7%); 3) neurons projecting to GPi and GPe (48%); 4) neurons targeting GPe only (10.7 %); and 5) neurons with axons that coursed toward the sriatum, but whose terminal arborization could not be visualized in detail (17.3%). Axons of the first two types bifurcated into rostral subthalamopallidal and caudal pallidonigral branches. However, the majority of STN axons had only a single branch that coursed rostrally toward the pallidum and striatum. These results reveal that, in contrast to current beliefs, the primate STN is not a monolithic entity. This nucleus harbors several subtypes of projection neurons, each endowed with a highly patterned set of collaterals. This organization allows STN neurons to exert a multifarious effect not only on the GPe, with which the STN is reciprocally connected, but also on the two major output structures of the basal ganglia, the SNr and the GPi. J. Comp. Neurol. 424:142–152, 2000.

Single‐axon tracing and three‐dimensional reconstruction of centre médian‐parafascicular thalamic neurons in primates

The axonal projections from the centre médian (CM)/parafascicular (Pf) thalamic complex in squirrel monkeys were studied after microiontophoretic injections of biotinylated dextran amine under electrophysiological guidance. A total of 29 axons connected to their parent cell body were entirely reconstructed from serial sections with a camera lucida. Our investigation shows that the CM and Pf nuclei in primates comprise three types of projection neurons: (1) neurons that innervate densely and focally the striatum; (2) neurons that arborize diffusely in the cerebral cortex; and (3) neurons that innervate both striatum and cerebral cortex. Striatal innervation of CM origin consists of dense clusters of axon terminals exhibiting pedunculated varicosities and forming oblique bands in the dorsolateral sector of putamen (sensorimotor striatal territory). The same type of striatal innervation occurs in the head of caudate nucleus (associative striatal territory) in cases of Pf‐labeled neurons. The CM neurons that target cerebral cortex arborize principally in motor and premotor areas, whereas Pf neurons innervate chiefly prefrontal areas. Cortical innervation from both nuclei is much more profuse in layers V and VI than in layer I. Our three‐dimensional reconstruction studies show that dendritic and axonal arborizations of CM neurons extend essentially along the sagittal plane. These results revealed that, in contrast to rodents where virtually all Pf neurons project to both striatum and cortex, the primate CM/Pf complex harbors several types of highly patterned projection neurons. As such, this complex might be considered as an integral part of the widely distributed basal ganglia neuronal system. J. Comp. Neurol. 481:127–144, 2005.

The axonal arborization of single nigrostriatal neurons in rats.

Neurons of the substantia nigra pars compacta (SNc) were iontophoretically injected with biotin dextran and their anterogradely labeled axons individually reconstructed from serial sagittal sections. Most nigrostriatal axons travelled directly to the striatum, where they branched abundantly. Other axons arborized profusely in various extrastriatal structures, including the globus pallidus, the entopeduncular and subthalamic nuclei, and branched only sparsely in the striatum. This heterogeneous organization of the nigrostriatal projection allows single SNc neurons to influence differently striatal neurons and to act directly upon extrastriatal components of the basal ganglia via a highly patterned set of collaterals.

Single-axon tracing study of corticostriatal projections arising from primary motor cortex in primates

The axonal projections arising from the forelimb area of the primary motor cortex (M1) in cynomolgus monkeys (Macaca fascicularis) were studied following microiontophoretic injections of biotinylated dextran amine under electrophysiological guidance. The microinjections were centered on layer V, and 42 anterogradely labeled corticofugal axons were reconstructed from serial frontal or sagittal sections with a camera lucida. Our investigation shows that the primate striatum receives both direct and indirect projections from M1. The direct corticostriatal projection is formed by axons that remain uniformly thin and unbranched throughout their sinuous trajectory to the ipsilateral striatum. They divide as they enter the dorsolateral sector of the postcommissural putamen, the so‐called sensorimotor striatal territory. The indirect corticostriatal projection derives from a thin collateral emitted within the corona radiata by thick, long‐range fibers that descend toward the brainstem. The collateral enters the putamen dorsomedially and remains unbranched until it reaches the dorsolateral sector of the putamen, where it breaks out into two to four axonal branches displaying small and equally spaced varicosities. Both direct and indirect corticostriatal axons branch moderately but occupy vast rostrocaudal striatal territories, where they appear to contact en passant several widely distributed striatal neurons. These findings reveal that, in contrast to current beliefs, the primate motor corticostriatal system is not exclusively formed by axons dedicated solely to the striatum. It also comprises collaterals from long‐range corticofugal axons, which can thus provide to the striatum a copy of the neural information that is being conveyed to the brainstem and/or spinal cord. J. Comp. Neurol. 496:202–213, 2006.

A re-evaluation of the current model of the basal ganglia

The current model of basal ganglia organization has been developed progressively over the last two decades in the light of key observations made at both experimental and clinical levels. This model has been highly successful in that it has stimulated a large amount of research in the field. However, several experimental and clinical findings that are at odds with the model have accumulated during the last decade. This paper reviews some of our own single-axon tracing studies in primates, which call for a re-evaluation of the current basal ganglia model.

Serotonin innervation of basal ganglia in monkeys and humans.

This review paper summarizes our previous contributions to the study of serotonin (5-hydroxytryptamine; 5-HT) innervation of basal ganglia in human and nonhuman primates under normal conditions. We have visualized the 5-HT neuronal system in squirrel monkey (Saimiri sciureus) and human postmortem materials with antibodies directed against either 5-HT, 5-HT transporter (SERT) or 5-HT synthesizing enzyme tryptophan hydroxylase (TPH). Confocal microscopy was used to compare the distribution of 5-HT and dopamine (DA; tyrosine hydroxylase-immunolabeled) axons in human, while the ultrastructural features of 5-HT axon terminals in monkey subthalamic nucleus were characterized at electron microscopic level. In monkeys and humans, midbrain raphe neurons emit axons that traverse the brainstem via the transtegmental system, ascend within the medial forebrain bundle and reach their targets by coursing along the major output pathways of the basal ganglia. These 5-HT axons arborize in virtually all basal ganglia components with the substantia nigra receiving the densest innervation and the striatum the most heterogeneous one. Although the striatum - the major basal ganglia input structure - appears to be a common termination site for many of 5-HT ascending axons, our results reveal that the widely distributed 5-HT neuronal system can also act directly upon neurons located within the two major output structures of the basal ganglia, namely the internal pallidum and the substantia nigra pars reticulata in monkeys and humans. This system also has a direct access to neurons of the DA nigrostriatal pathway, a finding that underlines the importance of the 5-HT/DA interactions in the physiopathology of basal ganglia.